Problem: Which of the following reactions will occur spontaneously as written?a) 2Cr3+ (aq) + 3Sn2+ (aq) ? 3Sn4+ (aq) + 2Cr (s)b) 3Fe2+ (aq) + Cr3+ (aq) ? Cr (s) + 3Fe3+ (aq)c) Sn2+ (aq) + Fe2+ (s) ? Sn4+ (aq) + Fe3+ (aq)d) 2Cr (s) + 3Fe2+ (s) ? 3Fe (s) + 2Cr3+ (aq)e) Sn4+ (aq) + Fe2+ (s) ? Sn2+ (aq) + Fe (s)

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FREE Expert Solution

We are asked to determine which of the given reactions will occur spontaneously as written.


Recall that the greater the E°cell of a redox reaction, the more likely the reaction will occur (more spontaneous reaction)

E°cell > 0 spontaneous in the forward direction
E°cell < 0 nonspontaneous in the forward direction


a) 2 Cr3+(aq) + 3 Sn2+(aq) → 3 Sn4+(aq) + 2 Cr(s)

Calculate E°cell.

 Sn4+(aq)  → Sn2+(aq)                        E°cell = 0.154 V                       anode

Cr3+(aq)  →  Cr(s)                               E°cell = – 0.74 V                   cathode


E°cell=E°cathode-E°anodeE°cell=-0.74 V -+0.154 VEcell=-0.74 V -0.154 V  

  E°cell = -0.894 V

E°cell < 0 → nonspontaneous


b)  3 Fe2+(s) + Cr3+(aq) + → Cr(s) + 3 Fe3+(aq)

Calculate E°cell.

 Fe3+(aq)  → Fe2+(s)                           E°cell = +0.771 V                       anode

Cr3+(aq))  → Cr(s)                              E°cell = -0.74                    cathode


E°cell=E°cathode-E°anodeE°cell=-0.74V -+0.771VEcell=-0.74V -0.771 V  

  E°cell = -1.511 V

E°cell < 0 → nonspontaneous


c) Sn2+(aq) + Fe2+(s) → Sn4+(aq) + Fe3+(aq)

  • Both species increased oxidation number
  • Both are just oxidation so this is automatically non-spontaneous. 


nonspontaneous


d)  2 Cr(s) + 3 Fe2+(s) → 3 Fe(s) + 2 Cr3+(aq)

 Calculate E°cell.

Cr(s)  →  Cr3+(aq)                               E°cell = -0.74 V                       anode

Fe2+(s) →  Fe(s)                                  E°cell = – 0.440V                   cathode


E°cell=E°cathode-E°anodeE°cell=-0.440 V --0.74 VEcell=-0.440 V +0.74 V  

  E°cell = 0.30 V

E°cell > 0 → spontaneous


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Problem Details

Which of the following reactions will occur spontaneously as written?

a) 2Cr3+ (aq) + 3Sn2+ (aq) ? 3Sn4+ (aq) + 2Cr (s)

b) 3Fe2+ (aq) + Cr3+ (aq) ? Cr (s) + 3Fe3+ (aq)

c) Sn2+ (aq) + Fe2+ (s) ? Sn4+ (aq) + Fe3+ (aq)

d) 2Cr (s) + 3Fe2+ (s) ? 3Fe (s) + 2Cr3+ (aq)

e) Sn4+ (aq) + Fe2+ (s) ? Sn2+ (aq) + Fe (s)